High purity aluminum hydroxide is a high value-added powdery product, with purity at 99.9%-99.9999%. High purity aluminum hydroxide is currently a new type functional and structural material widely used in the fields of chemical engineering, coatings, construction, transportation and electronic apparatus, etc. For high purity aluminum hydroxide used as the starting material to make high purity aluminum oxide, its content of metal element impurities (include silicon) should be no more than 0.0005%.
Currently the major methods to produce high purity aluminum hydroxide include seeding-separation method, carbon separation method, and precipitation method:
A) Seeding Separation Method: a method to make micro-powdery aluminum hydroxide, using a sodium chlorate solution as the starting material, adding aluminum hydroxide crystal seeds for a seeding-separation treatment, followed by separating solid from the starting liquid materials, and rinsing and drying the isolated solid to obtain the product.
B) Carbon Separation Method: a method to make aluminum hydroxide, using a sintering refined solution as the starting material for the first-phase carbon separation degradation with addition of crystal seeds, filtering the resultant slurry to obtain the first-phase aluminum hydroxide; and then mixing the first-phase aluminum hydroxide with the sintering refined solution for the second-phase carbon separation degradation with addition of crystal seeds. The resultant slurry from the second-phase carbon separation degradation with addition of crystal seeds is then washed with soft water, filtered, and dried to obtain the finished aluminum hydroxide product.
C) Precipitation Method: a method to make aluminum hydroxide, including the homogeneous precipitation synthesis method and heterogeneous precipitation azeotropic distillation method, using an aluminum salt solution and an alkaline solution as starting materials, adding a small amount of polymer protection agent, such as polyethylene glycol or polyvinyl alcohol, through a homogenous precipitation reaction in a homogenizing emulsification reactor to obtain an aluminum hydroxide suspension. The aluminum hydroxide suspension goes through post-treatment to yield a filter cake; the filter cake is added into an n-butanol azeotropic solvent; then azeotropic distillation of the resultant solution yields an aluminum hydroxide gel; and finally the resultant aluminum hydroxide gel is dried to yield aluminum hydroxide powders.
Aluminum oxide is an important chemical engineering starting material, and aluminum oxide with purity above 99.99% is called high purity aluminum oxide, having various superior properties, such as high purity, low surface area, high light absorption, strong resistance to corrosion, strong resistance to wear, tolerance of high temperature, high Mohs hardness, and good insulation, etc., and can be used in the manufacture of high temperature resistant material, crucible, porcelain, artificial gemstones, phosphor materials, high pressure sodium light, special porcelain, YAG laser crystal parts and integrated circuit boards, etc.
The purity of aluminum oxide is an important parameter affecting the quality of sapphire crystals, among which, the low purity of aluminum oxide will result in the following effects: first, it will cause defects of sapphire crystals such as dislocation, bubbles, inclusions and cracks. These defects can usually absorb, scatter, reflect, or diffract magnetic, light, sound, and electric energies generated inside the crystals or inputted from outside of the crystals, thus affecting overall quality of the sapphire crystals and properties of LED products. Second, high content of metal element impurities in aluminum oxide starting material will cause discoloring of the sapphire crystals. Therefore, the growth of high quality sapphire crystals has put higher requirements on the purity of the aluminum oxide, in general, with the content of metal element impurities (including silicon) being no more than 0.001%.
In the process of achieving the present invention, the inventors discovered that the existing techniques contain at least the following issues.
The metal impurity content in the aluminum hydroxide product is too high, with the content of common metal element impurities (including silicon) exceeding the 0.0005% requirement.
Aluminum oxide is usually prepared by the base method, acid method, acid-base combination method, and heat method, etc., and thus prepared industrial aluminum oxide contains relatively high concentration of other metal impurities, with purity generally low, which can only reach 99% purity. The existing techniques use de-ionized water, hydrochloric acid, and hydrofluoric acid to wash the aluminum oxide to remove some specific metal impurities, which cannot remove metal ions below 10 ppm; and the content of common metal element impurities (including silicon) after purification exceeds 0.001% in aluminum oxide, which cannot reach the purity requirement, and purification result is poor. In addition, the existing techniques involve complex processes to operate, with high energy consumption and high cost, and are likely to cause pollutions to the environment.